you believe there are no irreconcilable inconsistencies in the Bible. If you are the kind of uber-fundamentalist modern fanatic, you believe there are no inconsistencies at all. Of course, that also means you haven't actually read the thing.
The many contradictions, inconsistencies, and errors in the Bible are proof that the Bible can't possibly be the inerrant perfect document they believe it to be. This means that their God isn't the perfect God they believe him to be, because the Bible is His book. Or it means that the Bible is entirely the product of the human imagination, with a bit of history thrown in to make it more convincing.
And yes, Christians are taught to believe in the perfection of the Bible (which they know is perfect because the Bible says it is).
They get around the disagreements amongst the gospel accounts of Jesus' life by claiming the Bible is the accurate representation of what the witnesses said. This doesn't account for the differences between the two genealogies of Christ, nor does it explain why Christ even has a genealogy traced through Joseph if he was conceived by the Holy Ghost.
The key word is "irreconcilable." Reconciling the inconsistencies and contradictions of the Bible is part of what's called "apologetics."
this essay brings up some interesting points, including a curse on some of the kings in Joseph's lineage. Thebibleanswer guy claims it's cultural, and anyway the Luke genealogy is through Mary's family. Paul Carlson says that's baloney, since people of the time believed that sperm alone were the source of a baby and the woman was merely the incubator.
A Mormon site claims the Matthew lineage was about legal, not genetic, succession. Fair enough. Then they point out that Luke's version could apply to either Mary or Joseph since they were cousins. This seems to come from what Catholics call "tradition."
And you can interpret writings as human-inspired despite the Bible supposedly being inspired by God. If the reason for both lineages was to convince two groups of people of Christ's valid claim to be the next great King, then that makes them propaganda, not history.